Best solution for a material composed by various material (all final products)

I'm searching what's the best way to manage a group of materials that can get together into another material (for example, a trouser with a belt, the trouser alone and the belt alone, 3 different material codes but one of them it's other two together).

I need to be able to change from grouped material to the simple materials, and reverse process if it's neccesary. I have tried with a BOM but it needs a vendor, purchase order, managing the simple items to the MRP Area, etc... and all the components are final products (not raw materials).

I can't do a "material to material" change because it's not 1 to 1 relation.

You can follow for production order or process order and have BOM for one material(finished product) by keeping group materials( raw materials). Discuss with PP consultant for details of handling consumption of more than one materials to produce a single material.